This invention relates to varistors and varistor powders used in the manufacture of metal oxide varistors. In particular, this invention relates to a method for adding dopants to the varistor in such a way that the dopants occur only in the zinc oxide grains and not in the intergranular phase of a sintered metal oxide varistor. Even more particularly, this invention relates to a method of doping metal oxide varistor powders with certain trivalent impurities so as to increase the conductivity of the zinc oxide grains without having these dopants present in the intergranular phase of sintered metal oxide varistors.
Metal oxide varistors are nonlinear electrical devices exhibiting a current-voltage characteristic expressed by the equation EQU I=(V/C).sup..alpha.
where
I is the current flowing through the material,
V is the voltage across the material
C is constant which is a function of the physical dimensions of the device, its composition and the parameters of the processes employed to form the body, and,
.alpha. is a constant for a given range of current and is a measure of the nonlinearity of the resistance characteristic of the varistor.
For voltage values below the breakdown voltage, the device behaves like an ohmic resistor of very large value (typically approximately 10,000 M.OMEGA.), but when the breakdown voltage is exceeded, the device behavior is like that of a low resistance conductor. For a wide range of current values, the value of .alpha. is approximately constant. However, for certain large values of the varistor current the value of .alpha. is lower and there is a correspondingly high increase in the voltage across the device. This region of decreasing .alpha. value is referred to as the "upturn region". It is desirable to have this upturn region beginning at as high a current level as possible. In addition, it is desired to have the current level at which the varistor breakdown voltage occurs to be at as low a level as possible in order to minimize the problem of leakage current. A relatively high level of leakage current may not be a significant consideration in all varistor applications, but in those applications in which a standby voltage is present across the device, then the leakage current should be as small as possible to avoid unnecessary energy dissipation in the device. Thus, in general it is desirable that the range of current levels over which the varistor behaves nonohmically in accordance with the equation above, be as large as possible.
Certain trivalent dopants may be added to the varistor material which serve to increase the conductivity of the zinc oxide grains present in a sintered varistor. This increased conductivity has the effect of extending the upper limit of the current range at which the upturn region of the device is entered. However, in standard methods for adding this dopant to varistor materials, the dopants are added to the zinc oxide powder along with the other typical metal oxide additives. This method, however, results in the dopant, or dopants, being present in the intergranular phase of the sintered metal oxide varistor powder. The presence of such dopants in the intergranular phase, that is, between the sintered grains of zinc oxide, tends to increase the leakage current for the varistor. That is to say, when these dopants are added to the varistor powder in the usual fashion, the effect is simply to shift the voltage-current curve (as plotted in a log-log fashion) to the right without actually increasing the range of current values over which the varistor behaves nonohmically.